Owlvit memory leak fix (#18734)
Owlvit memory leak fix (#18734)

* fix memory leak
* fix typos
* use singular last hidden state variable names
* eliminate double call to self.owlvit to return last hidden states
* eliminate 2nd call to self.vision_model in OwlViTModel
huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers